The present invention concerns an alveolar structure, designed in particular to cover a cylinder or more generally an inwardly skewed surface, together with a process for producing such a structure.
Flat alveolar bodies (usually called honeycombs) are generally produced by stacking rectilinear sheets or strips which are partly glued to each other according to transversal zones having all transversal sides parallel to each other, the final product being obtained by traction, perpendicularly to the plane of sheets or strips, in order to obtain prismatic alveoles with a more or less regular hexagonal shape. The alveoles obtained have walls, the sides of which, determined by the assembly lines, are all parallel to each other.
Flat alveolar bodies can only be placed on a cylinder if the ratio of a thickness thereof to a diameter of the cylinder is very low. As soon as this ratio increases, significant stresses appear in the walls of the alveoles which may either buckle in a compression zone or split in a traction zone.
Similar products, presenting greater rigidity in a privileged direction, are realized from the assembling of preformed strips as they are not expansible. Such alveolar products are flat and, without undergoing deformations harmful to their mechanical properties, may only cover roughly flat surfaces.
Alveolar products are described in, for example, U.S. Pat. Nos. 2,815,795, 3,100,928; 3,139,375 and 3,259,961; in the French Pat. Nos. 2.131.029 and 2.131.068; and in the German patent application No. 3.210.320.
Manufactured flat, as can be clearly seen from FIG. 5 of the French Pat. No. 2.131.029, these products have the characteristic, which only enables them to be used on tubes, of not being assembled over their whole flat assembly surface, but only by a fillet weld or soldering or by a scalloper riveting which may possibly result in deformation of the initial assembly surfaces, thereby possibly reducing the traction part and increasing in compression with the result that stresses could result in a rupture at the time of mounting on the cylinder.
It is obvious that limitation of the assembly surface to one line results in stress concentrations in service, independent of the fact that such a structure, if realized using several sheets situated opposite each other but not perfectly joined, would very probably be subject to corrosion.
The process according to the invention avoids these drawbacks and enables structures of strong thickness to be realized which can be placed naturally with very low external forces and stresses limiting those of expansion, where the body is manufactured in a compressed state, on cylindrical walls whose local bending radius may be slight.
The alveolar structure according to the present invention is specifically designed to cover at least part of a cylindrical surface. This structure, which comprises alveoles demarcated by several walls, is characterized in that at least one alveole comprises at least one wall having at least two edges roughly parallel to each other and at leat one wall comprising at least two edges not parallel to each other, with each of these edges roughly parallel and not parallel to each other having an orthogonal length projection different from zero on a line perpendicular to the said cylindrical surface passing through the edge in question.
The alveolar structure according to the invention is obtained using curved strips, i.e. the strip or strips comprise at least one non-rectilinear part.
A structure comprising several alveoles according to the invention may be obtained using at least one curved strip comprising several zones disposed transversally with respect to the curved strip, with some of the zones corresponding to walls having edges roughly parallel and others corresponding to walls having edges which are not parallel.
Some of the zones may be assembly zones, each of which is designed to be joined with another assembly zone, while other zones may be free zones not used in assembly but connecting assembly zones to each other.
At least some of the assembly zones may correspond to walls having roughly parallel edges.
At least some of the free zones may correspond to walls having roughly parallel edges.
An alveole comprising at least one wall having two edges roughly parallel defining a first plane and at least one wall having two coplanar edges not parallel defining a second plane can be usefully disposed on the surface to be covered in such a way that the angle defined by the first plane with the direction defined by one of the cylinder generators is smaller than the angle defined by the second plane with this same direction.
The structure according to the invention could include one or more strips, possibly roughly helicoidal.
The structure according to the invention may include several strips, each of these shutting itself.
The alveole structure may include alveoles having a form intermediate between that of a truncated prism and that of a truncated pyramid.
The base of this alveole can be roughly hexagonal and possibly constitute a regular hexagon.
One would not be departing from the context of the present invention if the alveolar structure were designed to cover, at least in part, a surface defined by a revolution cylinder.
The alveolar structure according to the invention may include at least one wall belonging to a plane roughly perpendicular to the direction of the generator of the cylinder to be at least partly covered.
The alveolar structure according to the invention may include at least one wall disposed in a plane roughly parallel to the direction of a generator of the cylinder to be at least partly covered.
Therefore and specifically so, the process according to the invention includes assembling, according to a special geometry which will be better understood from the examples which follow, roughly helicoidal strips or a succession of rings, the assembled zones being limited by sides which are sometimes roughly parallel, sometimes not parallel, alternately or not according to the examples cited. The shape of the alveole may be regular or irregular and, according to the shapes selected for the alveoles, the product may be assembled flat and then expanded or be first preformed before assembly so as to provide a product which can be either very easily mouldable or, on the other hand, very rigid, according to the designer's wish.
The present invention also concerns a process for demarcating zones or walls with parallel sides using a curved strip so as to produce an alveolar structure. According to this process, a roughly longitudinal reference line is defined, then pairs of points are marked on the line in order to demarcate the walls of the alveoles. Each of the two parallel sides defining a wall with parallel sides passes through each of the points of a given pair and is parallel to a transversal straight line with a direction close to the perpendicular straight line or to the reference line or to the straight line joining the two points of the pair.
The transversal straight line may visibly pass close to the point defined, either through the middle of the curved segment defined on the reference line by the pair, or through the middle of the rectilinear segment joining the two points of the pair.
One would not be departing from the context of the present inventin by realizing a preshaping of the strip before assembly. In the event of the structure thus obtained being flattenable, it could be compressed in such a way as to facilitate the operation of placing the structure in position, this being effected by simple release of the compression forces.
Again, one would not be departing from the context of the present invention by assembling the curved strip before shaping.
In this case, it will, if required, be possible to compress the structure thus obtained after shaping, the setting up of this structure being effected in the same way as indicated previously.
Of course, the portion of the cylinder to be covered may be covered with the alveolar structure according to the invention, both on its convex side and its concave side.
The present invention will be better understood and its advantages be more apparent from the description which follows of special examples, by no means restrictive, illustrated by the figures annexed and in which: